There are a few places on eBay that supply keys for these old cars. You should not need blanks, and to get them cut as the key number is usually stamped on the lock assy. Magnette keys normally start with the prefix FP and then 3 numbers. Just checked, our ignition lock has the number stamped on the face where you put the key in. If the car has the original locks, the ignition, drivers door and the fuel flap locks will all be the same key, however the boot lock will be different.
Hope this helps.
